"I mean, if I look at the big picture, since the beginning of the year, I'm very pleased with how I'm playing," said Federer, who reached the final at Indian Wells and has collected an ATP-high 22 match wins this season; last year he won his 22nd match at Roland Garros - in June.

"It's a good start to the season, no doubt. So I want to keep it up now for Geneva when I play next week, and then clearly we will shift over to the clay finally after a long stretch on the hard courts.

"I just couldn't find my rhythm on the serve, which was surprising. Maybe the [cool, mid-60s] temperature had something to do with it. I haven't played that many matches in the dark.

"It's a bit frustrating, but Kei did well to stay with me. He was more consistent in the second and the third, which are the ones he won. To his credit.

"The second set just got away from me and the third was a tough battle. It was a tough end for me."

Nishikori will be playing in only his second ATP Masters 1000 semi-final, and his first in 2½ years. He advanced by breaking Federer five times, including in the final game.

"I thought I really played well, especially in the third set," Nishikori said. "I was hitting balls deep and striking well. Everything was going well."